Vaccination aims to stimulate the immune response to a specific pathogen in advance of infection. When an individual is exposed to that pathogen, a memory response is triggered which prevents the establishment of infection. Vaccines therefore stimulate the adaptive immune response which unlike innate immunity, is long lived and has memory. There are two major arms to the adaptive immune system; the humoral immunity which involves the development of antibodies that can bind virus particles and certain antibodies that can neutralize infection, and the cell mediated immunity that leads to the development of cytotoxic T-cells that kill infected cells exposing viral epitopes in the context of HLA class I, in this way eliminating infected cells.
The only disease which has been eliminated by virtue of a successful vaccination campaign is smallpox. A campaign is currently in progress to eradicate polio. Features of virus infections that can be eliminated by vaccination are that they; are caused by viruses with stable virus antigens (i.e. very low mutation frequency, few subtypes), lack a reservoir in other animal species, do not persist in the body once the infection is over, and where vaccination leads to long lasting immunity. Viruses such as polio and measles fulfill these criteria whereas viruses such as influenza virus, HCV, and HIV that vary their protein sequences do not. It is for this reason that new and alternate approaches are required to develop vaccines for these diseases.
Hepatitis C is a liver disease that results from infection with the hepatitis C virus (HCV). It can range in severity from a mild illness lasting a few weeks to a serious, lifelong illness. Hepatitis C is spread via blood; the most common form of transmission is through sharing needles or other equipment used to inject drugs. The infection can be either “acute” or “chronic”. Acute HCV infection is an asymptomatic, short-term illness that occurs within the first 6 months after someone is exposed to the hepatitis C virus. For most people, acute infection leads to chronic infection, which can result in long-term complications and even death.
It is estimated that 170 million people are infected with HCV worldwide, equating to approximately 3% of the global population. There are also approximately 3-4 million people who are infected every year; with an estimated 80% of these newly infected patients progressing to chronic infection.
HCV is an enveloped positive stranded ribonucleic acid (RNA) virus with a diameter of about 50 nm, belonging to the genus Hepacivirus in the family Flaviviridae that replicate in the cytoplasm of infected cells. The only known reservoir for HCV is humans, although the virus has experimentally been transmitted to chimpanzees. The natural targets of HCV are hepatocytes and possibly B-lymphocytes. As of 2008, six different genotypes and more than 100 subtypes of the virus are known. Replication occurs through an RNA-dependent RNA polymerase that lacks a proofreading function, which results in a very high rate of mutations. Rapid mutations in a hypervariable region of the HCV genome coding for the envelope proteins enable the virus to escape immune surveillance by the host. As a consequence, most HCV-infected people proceed to chronic infection.
The 6 genotypes of HCV have different geographical spread. The disease in the early stages is generally asymptomatic; the majority of patients with chronic infection eventually progress to complications such as liver fibrosis and cirrhosis, and, in 1-5% of cases, hepatocellular carcinoma.
HCV is the major cause of non-A, non-B hepatitis worldwide. Acute infection with HCV frequently leads to chronic hepatitis and end-stage cirrhosis. It is estimated that up to 20% of HCV chronic carriers may develop cirrhosis over a time period of about 20 years and that of those with cirrhosis between 1 to 5% is at risk to develop liver carcinoma.
Influenza remains a significant cause of mortality and morbidity worldwide. The World Health Organisation (WHO) estimates that seasonal epidemics affect 3-5 million people annually and result in 250,000-500,000 mortalities. Influenza is caused by viruses in the family Orthomyxoviridae which are negative stranded RNA viruses. The influenza virus exists as three types, A, B and C of which only A is associated with pandemics. Types A viruses are found in both humans and animals, particularly birds but also other mammals such as pigs. Type A viruses are further typed into subtypes according to different kinds and combinations of virus surface proteins. Among many subtypes, influenza A (H1N1) and A (H3N2) subtypes were circulating among humans in 2009. Influenza A and B are included in the seasonal vaccine, whereas influenza C occurs only rarely, and so it is not included in the seasonal vaccine. Type B viruses are human specific and Type C viruses cause a very mild disease. The genomes of Orthomyxoviruses are segmented. Influenzaviruses Types A and B have 8 segments whereas type C has seven. Pandemics may arise as a result of re-assortment of gene segments when two different type A viruses infect the same cell. There is no immunity in the population to this novel re-assorted virus. Three pandemics occurred in the twentieth century: “Spanish influenza” in 1918, “Asian influenza” in 1957, and “Hong Kong influenza” in 1968. The 1918 pandemic killed an estimated 40-50 million people worldwide. Subsequent pandemics were much milder, with an estimated 2 million deaths in 1957 and 1 million deaths in 1968. In June 2009 the WHO declared a pandemic from influenza virus H1N1 (swine flu) which was declared over in August 2010.
Human papillomaviruses are made up of a group of DNA viruses in the family Papillomaviridae which infect the skin and mucous membranes. Two groups which are derived from more than 100 different identified subtypes are the main cause for clinical concern: those causing warts (both benign and genital warts), and a group of 12 “high risk” subtypes that can result in cervical cancer. This latter group has been attributed as a contributory factor in the development of nearly all types of cervical cancer. Worldwide, cervical cancer remains the second most common malignancy in women, and is a leading cause of cancer-related death for females in developing countries. HPV 16 and 18 have been mainly associated with cervical cancer; however, the virus is also a cause of throat cancer in both men and women. HPV is transmitted through contact and enters the skin through abrasions. An abortive infection, where only the early proteins are expressed is associated with cancer development.